1. Field of the Invention
The present invention generally relates to semiconductor devices, and more particularly to a fabrication process of a high power semiconductor device having the PHS (plated heat sink) structure. Further, the present invention relates to a semiconductor device fabricated according to such a fabrication process.
2. Description of the Related Art
In high power semiconductor devices, it is necessary to dissipate heat as efficiently as possible. For this purpose, such high power semiconductor devices use a semiconductor substrate having a thickness reduced as much as possible, in combination with the so-called PHS (plated heat sink) structure in which a metal layer is provided on the rear side of the substrate as a heat sink. Such a PHS structure is also useful in high speed semiconductor devices such as GaAs FET or HEMT for minimizing the length of the ground conductor pattern. In such semiconductor devices, through-holes are formed in a substrate of the device in correspondence to the source region, and the source electrode is connected to a grounded metal layer provided on the rear side of the substrate via the through-holes. Thereby, the problem of reduced gain, caused by the inductance of the conductor pattern that connects the source electrode to the ground, is successfully eliminated.
When fabricating a semiconductor device having the PHS structure, semiconductor chip patterns are formed on a surface of a semiconductor substrate that has an ordinary thickness. Next, the substrate is turned over and fixed upon a slab such as a glass substrate by means of wax, such that the front surface of the substrate faces the glass substrate. In this state, the rear side of the semiconductor substrate is ground uniformly by means of a back-grinder tool and diamond abrasives. Further, a chemical etching process is applied to reduce the thickness of the substrate to a thickness less than 50 .mu.m. After such a process, a resist pattern is provided on the rear side of the semiconductor substrate in correspondence to an electrode pattern on the front side of the substrate, and a plating is conducted on the rear side of the substrate while using the resist pattern as a mask to form an electrode pattern on the rear side of the substrate. After the individual semiconductor devices are formed as such, the substrate is divided into respective chips by conducting a dicing process. Typically, the substrate is cut by means of a dicing saw or etched along a dicing line defined on the surface of the substrate.
When fabricating a semiconductor device having a through-hole in the substrate, the substrate upon which the semiconductor devices are formed is mounted upon a slab or support substrate similarly as before, and the resist pattern is formed on the rear side of the semiconductor substrate so as to expose the region in which the through-hole is to be formed. Further, the resist pattern is formed to expose the foregoing dicing line. By conducting an etching process, the semiconductor substrate is divided into semiconductor chips separated from each other, and the through hole is formed in each of the semiconductor chips at the same time. When the etching is completed, the semiconductor chips are held upon the foregoing glass substrate by a wax, and an electrode is formed on the exposed rear side of the substrate by a plating process so as to fill the through-hole, while simultaneously protecting the dicing region by a resist pattern. The electrode thus formed also acts a heat sink. After the formation of the rear electrode, individual semiconductor chips are separated from the glass substrate by dissolving the wax by a suitable solvent.
In the foregoing fabrication process, it should be noted that the completed semiconductor chips are in alignment on the glass substrate as long as the chips are fixed thereupon by means of wax. On the other hand, once the wax is dissolved, the alignment of the chips is lost and the semiconductor chips are intermixed. Thereby, the use of an automatic assembling process, such as the one that uses an automatic chip-bonder apparatus, makes it difficult for constructing the semiconductor device in the form of semiconductor package.
In the foregoing processes, it should be noted that the front surface of the substrate is contacted with the glass substrate in a state in which the chips are aligned on the glass substrate, and it is the rear side of the chip, which carries the foregoing heat sink electrode, that is exposed for accessing from the outside. As a result, picking up of the semiconductor chips and assembling the same into a semiconductor package inevitably becomes a complex process, and because of this, there has been difficulty in the mass production of the semiconductor device having the PHS structure.